Preparation of furan compounds

ABSTRACT

A process for preparing a furan compound by the catalytic oxidation of a diolefin such as butadiene or a halo-substituted alkene is provided. In this process, the starting material is contacted with an aqueous medium having a pH less than about 2 containing (1) iodide ion, (2) a mixture of cuprous and cupric ions, and (3) a solubilizing agent for cuprous ion such as an alkali metal halide. Optionally, an oxygen-containing gas such as air can also be contacted with the aqueous medium. In one embodiment, the starting material is contacted with an aqueous solution of the aforesaid ingredients in one reaction zone to prepare furan and a portion of the solution is passed to a second reaction zone where cuprous ion is oxidized to cupric ion with air and then the air-treated solution returned to the first reaction zone.

This is a continuation of application Ser. No. 024,098, filed Mar. 26,1979, now abandoned.

DESCRIPTION

2. Technical Field

This invention relates to processes for the preparation of furancompounds, catalysts useful in the processes, and more particularly toprocesses for the preparation of furan compounds from diolefins andalkenes, particularly butadiene.

2. Background Art

Furan is a chemical useful in furan resins and, more importantly, as araw material for the manufacture of tetrahydrofuran. However, furantoday is prepared from natural pentose contained in corn or oat hullsthrough furfural as an intermediate. To reduce the cost oftetrahydrofuran, it is produced today from acetylene and formaldehydethrough 1,4-butynediol and 1,4-butanediol as intermediates. While thisis a satisfactory process, it is moderately complex in the number ofsteps required to reach tetrahydrofuran as the final product. Moreimportantly, however, acetylene is becoming more expensive due to energyinefficiencies involved in its manufacture.

There have been attempts over the years to produce furan directly by thecatalytic oxidation of butadiene. These attempts have generally beenunder harsh processing conditions, e.g., at temperatures higher thanabout 375° C., which result in overoxidation to carbon oxides and furandecomposition. Such high temperature, vapor phase processes areexemplified by U.S. Pat. Nos. 3,238,225; 3,716,545; 3,775,508;3,864,279; 3,906,009; 3,912,763; 3,928,389 and 4,026,820.

A process for preparing furan by oxidation of butadiene with molecularoxygen at lower temperatures (40°-150° C.) either in a vapor phasereaction or a liquid phase reaction is described in Japanese PatentApplication Publication No. 52-77049 dated June 29, 1977. In one aspectof the process described therein, a palladium salt and a thalium orindium salt are dissolved in acidified water and then butadiene andoxygen are passed through the solution. A similar process is describedin Russian Pat. No. 265119 dated June 24, 1970. In this process,butadiene (or butadiene and air) is passed through an acidic, aqueoussolution of cupric chloride and palladium chloride at a temperature of60°-110° C. Cuprous chloride can be used in place of palladium chloride.Both of these processes suffer deficiencies of impractical rates ofreaction and excessively low reaction life.

DETAILED DESCRIPTION

According to the present invention there is provided an improved processfor preparing a furan compound by the oxidation of a diolefin or a mono-or dihalo-substituted alkene, wherein the improvement comprisescontacting in a reaction zone (A) a diolefin of the formula: ##STR1##where each R is H or an alkyl group of 1-4 carbon atoms, and

each R₁ is H, a halide or an alkyl group of 1-4 carbon atoms,

with the proviso that the total number of carbon atoms does not exceed8,

or (B) a mono- or dihalo-substituted alkene of 4-8 carbon atoms, with anaqueous medium having a pH less than about 2 containing (1) iodine fromelemental iodine or an iodine-containing compound, (2) copper having anaverage oxidation state between 1 and 2, and (3) a solubilizing agentfor cuprous ion which is soluble in water and forms a water-solublecomplex with cuprous ion.

In one embodiment of the present invention there is provided an improvedprocess for preparing a furan compound by the catalytic oxidation of adiolefin, wherein the improvement comprises (1) continuously contactingin a first reaction zone a diolefin of the formula: ##STR2## where eachR is H or an alkyl group of 1-4 carbon atoms, and

each R₁ is H, a halide or an alkyl group of 1-4 carbon atoms,

with the proviso that the total number of carbon atoms does not exceed8,

with an aqueous medium having a pH less than about 2 containing (a)iodine from elemental iodine or an iodine-containing compound, (b)copper having an average oxidation state between 1 and 2 and (c) asolubilizing agent for cuprous ion selected from the group consisting ofat least one of a metal halide, ammonium halide, halogen acid andorganic solvent; (2) continuously removing reaction gases from the firstreaction zone, removing furan compound product therefrom and returningunreacted diolefin to the first reaction zone; (3) continuously removingaqueous medium from the first reaction zone and passing it to a secondreaction zone; (4) continuously contacting an oxygen-containing gas withthe aqueous medium in the second reaction zone to oxidize cuprous ion inthe aqueous medium to cupric ion; and (5) continuously returningoxygen-treated aqueous medium to the first reaction zone.

The present process for the preparation of a furan compound such asfuran from a diolefin or a mono- or dihalo-substituted alkene uses acatalyst system comprising a mixture of cuprous and cupric ions, iodineand a solubilizing agent for cuprous ion contained in an aqueous medium.The system should contain at least 20 moles per liter of water. Thisprocess, which can be conducted at low temperatures and pressures,produces the furan compound at good conversions and yields and atpractical rates of reaction.

A 1,3-diolefin or a mono- or dihalo-substituted alkene of 4-8 carbonatoms which forms the 1,3-diolefin in situ under the reaction conditionsare used as the starting materials in the present process. In thefollowing paragraphs these chemicals will be referred to as "startingmaterials."

The useful 1,3-diolefins have the formula: ##STR3## where each R is H oran alkyl group of 1-4 carbon atoms (preferably methyl), and

each R₁ is H, an alkyl group of 1-4 carbon atoms (preferably methyl) ora halide such as chloro or iodo (preferably chloro),

with the proviso that the total number of carbon atoms does not exceed8, preferably does not exceed 5.

Illustrative diolefins are 1,3-butadiene; 1,3-pentadiene;chloroprene(2-chloro-1,3-butadiene); isoprene(2-methyl-1,3-butadiene);2-iodo-1,3-butadiene; 1,3-hexadiene; 2,4-hexadiene;2,3-dimethyl-1,3-butadiene; 3,4-dimethyl-2,4-hexadiene; 4,6-octadiene;and 1,3-octadiene. Of these diolefins, the first four listed arepreferred due to commercial availability, with 1,3-butadiene being mostpreferred. Mixtures of diolefins can be used if desired.

Illustrative of the useful mono- or dihalo-substituted alkenes are:1-chloro-2-butene; 3-chloro-1-butene; 1-iodo-2-butene;1-chloro-2-pentene; 1-chloro-2-hexene; 3-chloro-1-pentene and1-bromo-2-pentene. The preferred halo-substituted alkenes are thebutenes with the most preferred being crotyl chloride; and if desired,mixtures can be used.

When used in the process of the invention, the starting material can beused undiluted, mixed with a gas inert to the reaction, such asnitrogen, carbon monoxide or carbon dioxide, or used with anoxygen-containing gas such as air. In a preferred embodiment of theinvention, the starting material is mixed with the oxygen-containinggas, e.g., at about a 20-50 percent by volume starting material when theoxygen-containing gas is air. The starting material is then contactedwith the above-mentioned aqueous medium in a reaction zone, underreaction conditions suitable for conversion to furan compounds,preferably with a molecular oxygen-containing gas.

The aqueous medium with which the starting material is contacted shouldhave a pH less than about 2, as measured by any known type of pHmeasuring device. A pH meter with glass electrodes is typically used.Furan production is increased when the pH is less than about 0.5 and itis preferred that the pH be about 0.0 or less.

However, measurement of pH by glass electrodes in the aqueous solutionsof copper salts which are used in the process of the invention does notaccurately measure the molar concentrations of hydrogen ion. Forexample, the pH of a solution which is 0.1 Normal is hydrochloric acidand contains the concentrations of copper salts which are exemplified isbelow 0 when measured with a glass electrode. The molar concentrationsof hydrogen ions in the mixes may be determined by titrations of aliquotsamples dissolved in 10-fold quantities of water with standard basesolutions. Standard techniques for determining the end-points ofacid-base titrations may be used, i.e., by indicators, such as Congo redor methyl orange, or with a pH meter.

In the present process, the molarity of the hydrogen ion will preferablybe greater than about 0.05, preferably in the range of about 0.1-1.0.

It is preferred that the aqueous medium be an aqueous solution withwater being used as the only solvent; however, the term aqueous mediumalso means aqueous solutions in which the water is diluted withhydrophilic solvents such as acetic acid, sulfolane, acetonitrile,dioxane, and the like. Aqueous medium also includes those aqueoussolutions in which an organic solvent is used as a cuprous ionsolubilizing agent as discussed later.

The aqueous medium used in the present process contains (1) iodine, (2)a copper redox catalyst, i.e., a mixture of cuprous and cupric ions, and(3) a solubilizing agent to aid in keeping cuprous ion in solution.

Iodine is thought to be present in the aqueous medium as iodide ionwhich is typically added as elemental iodine or as an alkali metaliodide, preferably sodium iodide or potassium iodide. Since very littleiodine is needed, any iodine-containing compound can be used which is atleast partially soluble in the aqueous medium. Illustrativeiodine-containing compounds are lithium iodide, calcium iodide, cuprousiodide, ferrous iodide, potassium iodate and hydiodic acid, and organiciodides such as methyl iodide and ethyl iodide. Of these, iodine fromelemental iodine or alkali metal iodides are preferred. Theconcentration of iodine in the aqueous medium will normally be in therange of about 1×10⁻¹² -0.5 gram mole per liter, preferably in the rangeof about 0.001-0.2 gram mole per liter.

The metallic component of the catalyst of the invention is copper. Thecopper in the aqueous medium has an average oxidation state between 1and 2, i.e., the copper is a mixture of cuprous ion and cupric ion. Anycopper compound soluble in the aqueous medium can be used, althoughcopper halides such as the chlorides and bromides are preferred.Especially preferred is a mixture of cupric chloride and cuprouschloride even though either one alone can be added to the aqueousmedium, in which event one can very quickly obtain a mixture of the twocopper ions either through oxidation of cuprous to cupric or reductionof cupric to cuprous. The total copper concentration in the aqueousmedium will usually be in the range of about 0.1-10 gram moles perliter, and normally about 0.5-3 gram moles per liter. Under preferredoperating conditions, there will be a ratio of cupric ion to cuprous ionof 100:1 to 1:2, preferably 10:1 to 1:1. Illustrative copper compoundsthat can be used are halides of copper such as cupric chloride, cupricbromide, cuprous chloride, cuprous bromide and cuprous iodide; coppersalts of organic acids, which may be carboxylic acids, such as acetic,propionic, pivalic, formic, succinic or adipic acids, fluorinatedcarboxylic acids, such as trifluoroacetic acid, sulfonic acids, such asmethane sulfonic acid, benzenesulfonic acid and p-toluenesulfonic acidor fluorinated sulfonic acids, such as trifluoromethyl sulfonic acid; orsalts of inorganic acids, such as cupric sulfate, cupric nitrate andcupric tetrafluoroborate.

In order to keep cuprous ion in solution, a solubilizing agent is used.A useful solubilizing agent is any inorganic or organic compound whichis soluble in water and tends to form a water-soluble complex withcuprous ion. While alkali metal halides, alkaline earth metal halides,ammonium halides and halogen acids are preferred, other metal halidessuch as palladium halides and iron halides, and organic solvents canalso be used. By halides it is meant the chlorides and bromides, andpreferably the chlorides. Illustrative organic compounds are (1) organicnitriles including aliphatic nitriles such as acetonitrile,succinonitrile, and propionitrile and aromatic nitriles such asbenzonitrile; (2) carboxylic acids such as acetic acid; (3) thiocyanatessuch as sodium thiocyanate; and (4) aromatic amines or theirhydrochlorides such as tetramethylethylenediamine. It will be within theskill of the art to select a particular solubilizing agent and theappropriate amount to use. Especially preferred solubilizing agents aresodium chloride, calcium chloride and ammonium chloride.

The concentration of the solubilizing agent is typically in the range ofabout 0.01-5 gram moles per liter, preferably about 0.5-3 gram moles perliter.

The process of the invention can be carried out at a temperature in therange of about 50°-125° C., preferably 75°-105° C. and most preferablyabout 95°-103° C. As would be expected, rates of furan production arereduced at the lower temperatures. Reaction pressures are typically inthe range of about 0.1-10 atmospheres, preferably about 1-3 atmospheresand most preferably at atmospheric pressure. It is the startingmaterial's partial pressure in the gas stream contacted with the aqueousmedium that can determine a particular pressure used.

The starting material flow rate through the aqueous medium does notappear to be critical. As will be apparent, the flow rate should not beso fast as to give inadequate contact time between the starting materialand aqueous medium or so slow as to enable the resulting furan producttime to decompose or polymerize. It is preferred that the aqueous mediumbe agitated either mechanically or by good gas dispersion in the aqueousmedium, and the reaction off-gases containing furan product be removedfrom the reaction vessel promptly. The optimum contact time between thestarting material and aqueous medium depends on many factors and isreadily determined by one skilled in the art.

Since cupric chloride is very corrosive, the reactor for carrying outthe process of the invention should be made of a material which is notcorroded by the aqueous medium. Illustrative materials are glass orceramic-lined metals, titanium, tantalum-clad metals, impregnatedgraphite tubes and the like.

In a preferred embodiment of the invention, an oxygen-containing gas iscontacted with the aqueous solution, along with the starting material tooxidize cuprous ion formed to cupric ion. Typically, the startingmaterial and oxygen-containing gas are mixed and then passed through theaqueous medium, although they can be fed as two separate gas streams.The oxygen-containing gas employed can be molecular oxygen as such, ormolecular oxygen utilized with a diluent inert to the reaction such asnitrogen and the like. Typical molecular oxygen-containing gases areair, which is preferred, flue gases or synthesis gases which containresidual oxygen, and any source of molecular oxygen which is at leastessentially free of contaminants which would be detrimental to thedesired reaction. The amount of oxygen-containing gas used is sufficientto provide about 1-2 moles of molecular oxygen per mole of furancompound prepared.

Another preferred embodiment of the invention is a two-stage processwherein furan is produced in a first stage by contacting the startingmaterial with the aqueous solution and cuprous ion is oxidized to cupricion in a second stage by contacting the aqueous solution containingexcess cuprous ion with the abovedescribed oxygen-containing gas. Thistwo-stage process can be conducted either intermittently in one reactionvessel or continuously in two reaction vessels.

In intermittent operation, the starting material is contacted with theaqueous medium until such time as furan production starts to decrease,the flow of starting material is stopped and then the oxygen-containinggas is contacted with the aqueous medium until essentially the originalcupric ion concentration is reached. The course of the reaction can bemonitored by following the acidity of the aqueous medium or by measuringthe cuprous and cupric ion content.

In continuous operation, furan compound is produced in a first reactionvessel by continuously contacting starting material with the aqueousmedium. The reaction gases, including unreacted starting material, inertgases and reaction products in addition to the desired furan compoundare continuously removed from the reaction vessel, furan compoundremoved therefrom by conventional techniques and any unreacted startingmaterial recovered and recycled to the first reaction vessel along withmake-up starting material. The aqueous medium is continuously circulatedas a working solution between the first reaction vessel, where the furancompound is produced, and a second reaction vessel, where anoxygen-containing gas (air) is contacted with the now cuprous ion-richaqueous medium to oxidize cuprous ion therein to cupric ion. Theoxygen-treated aqueous medium enriched with cupric ion is continuouslyreturned to the first reaction vessel.

In the examples which follow, conversion of starting material, e.g.,butadiene, is the mole percent of starting material fed which isconverted to products. The furan yield is the mole percent of productwhich is furan. When operated under optimum conditions, the presentprocess can achieve a conversion of starting material, e.g., butadiene,of about 10-90 percent and a furan yield of about 70-95 percent.

In the Examples samples for gas chromatographic analyses were collectedin 1 ml Carle sampling loops from the product stream. The samples wereinjected by Carle sampling valves onto 10'×1/8" columns of "Porapak N"for determination of their air, carbon dioxide, butadiene and furancontents. Analyses were carried out at 175° C. with helium carrier gasat 25 ml/min. The areas of the peaks in the chromatograph were convertedto volume percents of components using factors determined bycalibrations with known quantities of components.

EXAMPLE 1

A mixture of 50 percent by volume of butadiene and 50 percent by volumeof nitrogen was fed at a rate of 80 ml/min through a fritted glass discat the bottom of a glass reactor into 100 ml of an aqueous solution,having an initial pH of about 0.5, which has 2 molar in cupric chloride,1 molar in cuprous chloride, 1.7 molar in sodium chloride, 0.24 molar inpotassium iodide and 0.06 molar in hydrochloric acid. The solution wasmaintained at a temperature of 95° C. by an external heating jacket andwas stirred with a vane disc stirrer. After 0.5 hour of operation, thegaseous product stream contained 10.9 percent by volume furan, asanalyzed by gas chromatography (GC), produced at a rate of 8.7 ml/min.The conversion of butadiene to products was 20 percent and the yield offuran was 95 percent.

As a control without added iodine, butadiene was fed at a rate of 40ml/min into the above stirred glass reactor which contained 100 ml of anaqueous solution having an initial pH at or below 0 which was 2 molar incupric chloride, 2 molar in cuprous chloride and 4 molar in lithiumchloride. The temperature was maintained at 95° C. After 1 hour ofoperation, the gaseous product stream contained 3 percent by volumefuran analyzed by GC. Furan was produced at a rate of 1.2 ml/min.

EXAMPLE 2

Example 1 was repeated except the aqueous solution was 0.03 molar inpotassium iodide. After 1 hour of operation, furan was being produced ata rate of 7 ml/min.

EXAMPLE 3

A mixture of 50 percent by volume of butadiene and 50 percent by volumeof nitrogen was fed at a rate of 120 ml/min through a fritted glasscylinder located under a vaned disc stirrer in a 1 liter baffled flaskreactor. The flask contained 400 ml of an aqueous solution at 95° C.,having an initial pH of 2. The solution was 2.05 molar in cupricchloride, 0.5 molar in cuprous chloride, 0.86 molar in sodium chlorideand 0.0075 in potassium iodide. Furan production in the gaseous productstream was followed by GC. The results are shown in Table I.

                  TABLE I                                                         ______________________________________                                        Reaction                                                                      Time     pH of     Vol. % Furan Furan Prod.                                   Min      Solution  in Exit Gases                                                                              ml/min                                        ______________________________________                                        15       2.0       0.2          0.22                                          30       0.0       1.7          2.0                                           45       -0.3      5.4          6.4                                           60       <0.0      8.1          9.7                                           75       <0.0      10.7         12.8                                          90       <0.0      11.3         13.6                                          ______________________________________                                    

The conversion of butadiene was 22-30 percent and the yield of furan was88 percent.

EXAMPLE 4

A gaseous stream of 50 percent by volume of butadiene in nitrogen wasfed at a rate of 100 ml/min to the reactor of Example 1 containing 100ml of an aqueous solution which was 2 molar in cupric chloride, 1 molarin cuprous chloride, 2 molar in calcium chloride and 0.04 molar inelemental iodine. The solution had an initial pH less than 2 and wasmaintained at a temperature of 95° C. The gaseous product streamcontained 6.2 percent by volume of furan, as analyzed by GC, after 0.5hour of operation. Furan production was at a rate of 6.2 ml/min.

EXAMPLE 5

Example 1 was repeated except the aqueous solution for 2 molar in cupricchloride, 1 molar in cuprous chloride, 2 molar in ammonium chloride and0.12 molar in potassium iodide and had an initial pH of about 0.0. Afterabout one hour of operation, furan was produced at a rate of 4.6 ml/min.

EXAMPLE 6

Example 1 was repeated except the aqueous solution was 2 molar in cupricchloride, 0.5 molar in cuprous chloride, 5.7 molar in lithium chlorideand 0.12 molar in potassium iodide. Furan was produced at a rate of 1.7ml/min after 0.5 hour of operation.

EXAMPLE 7

This example illustrates rates of furan production at increasing levelsof cupric ion concentration in the aqueous solution.

A gaseous stream of 50 percent by volume butadiene in nitrogen was fedat a rate of 80 ml/min into reaction flask of Example 3 containing 400ml of an aqueous solution, having initial pH less than 2, which was 0.5molar in cupric chloride, 1.5 molar in cuprous chloride, 3 molar insodium chloride and 0.3 molar in potassium iodide. At a reactortemperature of 100° C., the gaseous product stream contained 5 percentby volume of furan, as measured by GC, produced at a rate of 4 ml/min.The conversion of butadiene was 8 percent and the yield of furan was 85percent.

The reaction was stopped, the cupric chloride concentration of theaqueous solution was increased to 1.2 molar, and then the flow of thebutadiene-nitrogen gas stream resumed. The aqueous solution had a pHless than 0 and the reaction temperature was maintained at 103° C. Afterone hour of operation, the gaseous product stream was analyzed by GC at15.1 percent by volume furan at a production rate of 12 ml/min. Theconversion of butadiene to product was 32 percent and the yield of furanwas 87 percent.

Again the reaction was stopped and the cupric chloride concentration ofthe aqueous solution increased, this time to 1.53 molar. The flow of thebutadiene-nitrogen gaseous stream was resumed. At a reactor temperatureof 104° C. and a solution pH less than 0, the gaseous product streamcontained 22 percent by volume furan produced at a rate of 17.6 ml/minafter 0.5 hour of operation. The conversion of butadiene was 55 percentand the yield of furan was 80 percent.

EXAMPLE 8

A gaseous stream of 46 percent by volume of butadiene in nitrogen wasfed at a rate of 65 ml/min into 250 ml of an aqueous solution containedin a stirred reaction flask. The aqueous solution was initially at a pHless than 2 and 2.4 molar in cupric chloride and 0.05 molar in potassiumiodide. Two ml of tetramethylethylenediamine were added to the aqueoussolution. At a reaction temperature of 90°-95° C., furan was produced ata rate of 3.9-5.2 ml/min to give a furan concentration in the gaseousproduct stream of 6-8 percent by volume as analyzed by GC.

EXAMPLE 9

A gaseous stream of 50 percent by volume butadiene in nitrogen was fedat a rate of 150 ml/min into the reactor of Example 3 containing 400 mlof an aqueous solution which was 1 molar in cupric chloride, 0.5 molarin cuprous chloride, 1 molar in calcium chloride and 0.06 molar inpotassium iodide. The acid content of the aqueous solution at the startof the reaction, as measured by titration with standard base, was 0.1molar. During the course of the reaction, the acidity of the solutionwas followed by base titration of aliquot samples and the cupric ionconcentration was calculated on the basis of the loss of one Cu⁺⁺ ionfor each H⁺ formed. The temperature of the aqueous solution wasmaintained at 75° C. Furan production was followed by GC with the rateof production shown in Table II.

                  TABLE II                                                        ______________________________________                                        Reaction                                                                      Time      Soln. Conc. - mole/l                                                                          Furan Prod.                                         Min       H.sup.+  Cu.sup.++  Vol. % ml/min                                   ______________________________________                                         25       .120     .98         0.40  0.6                                       70       .218     .88        0.78   1.2                                      100       .305     .80        1.2    1.8                                      135       .350     .75        1.4    2.1                                      200       .470     .63        1.9    2.9                                      230       .530     .57        1.9    2.9                                      ______________________________________                                    

The reaction was stopped and air was fed into the aqueous solution for30 min at a rate of 150 ml/min to oxidize the excess cuprous chloride inthe solution (formed by the reduction of the cupric chloride during thereaction). After the cupric chloride concentration had been restored toits initial concentration, the flow of the butadiene-nitrogen gaseousmixture was resumed at the rate of 150 ml/min, but the aqueous solutionwas then maintained at a temperature of 85° C. Furan production wasfollowed as before with the rate of production as a function of solutionacidity and cupric ion content shown in Table III.

                  TABLE III                                                       ______________________________________                                        Reaction                                                                      Time      Soln. Conc. - mole/l                                                                          Furan Prod.                                         Min       H.sup.+  Cu.sup.++  Vol. % ml/min                                   ______________________________________                                         24        .175     .925      1.1    1.7                                       65        .355    .75        2.5    3.8                                      110       .50      .60        5.1    7.7                                      130       .59      .51        5.1    7.7                                      145       .63      .47        4.4    6.6                                      ______________________________________                                    

The reaction was again stopped to oxidize excess cuprous chloride to itsinitial cupric chloride content by feeding air into the solution at arate of 150 ml/min. After the cupric chloride concentration had beenrestored, the flow of the butadiene-nitrogen mixture was resumed at 150ml/min, but the solution temperature was maintained at 95° C. Furanproduction was followed as before with the rate of production as afunction of solution acidity and cupric ion content shown in Table IV.

                  TABLE IV                                                        ______________________________________                                        Reaction                                                                      Time      Soln. Conc. - mole/l                                                                          Furan Prod.                                         Min       H.sup.+  Cu.sup.++  Vol. % ml/min                                   ______________________________________                                        20         .245     .855       .9    1.4                                      35         .335     .765      2.7    4.1                                      50        .44      .66        6.5    9.8                                      65        .53      .57        9.8    14.7                                     80        .67      .43        9.0    13.5                                     95        .73      .37        6.5    9.8                                      ______________________________________                                    

EXAMPLE 10

The reactor of Example 3 was charged with 400 ml of an aqueous solutionwhich was 1 molar in cupric chloride, 0.5 molar in cuprous chloride, 1molar in calcium chloride and 0.06 molar in potassium iodide. Mixturesof butadiene (BD) in air, as shown in Table V below, were fed into thesolution in the reactor at a rate of 150 ml/min. The progress of thereaction is shown in Table V.

                  TABLE V                                                         ______________________________________                                        Reaction        Soln.                                                         Time   Temp.    Acidity  BD in Feed                                                                             Furan Prod.                                 Hrs.   °C.                                                                             Mole/l   Vol. %   Vol. % ml/min                               ______________________________________                                        0.25   95       .12      46.6     1.5    2.3                                  1.0    96       .26      40.0     8.0    12.0                                 1.6    98       .28      40.0     8.8    13.2                                 2.5    96       .21      40.0     7.0    10.5                                 3.5    98       .31      40.0     8.2    12.3                                 5.5    98       .39      40.0     8.1    12.2                                 6.0    98       .37      40.0     8.9    13.4                                 6.5    99       .37      40.0     9.0    13.5                                 9.0    99.5     .35      53.0     10.2   15.3                                 10.0   98.5     .25      40.0     5.4    8.1                                  11.0   97       .21      40.0     5.3    8.0                                  11.5   97       .15      53.3     4.0    6.0                                  12.5   98       .20      53.3     7.1    10.7                                 13.5   98       .29      53.3     9.1    13.7                                 ______________________________________                                         *Run stopped for the day and restarted the next morning.                 

This example shows a one-step reaction for the preparation of furansimultaneously with the oxidation of cuprous chloride to cupricchloride.

EXAMPLE 11

A gaseous mixture of 50 percent by volume butadiene in nitrogen was fedat a rate of 100 ml/min into the reactor of Example 1 containing 100 mlof an aqueous solution which was 1 molar in cupric bromide, 0.5 molar incuprous bromide, 1 molar in sodium bromide and 0.12 molar in potassiumiodide. The pH of the solution was 0.0 and the temperature duringreaction was maintained at 95° C. After 45 minutes of operation, thefuran concentration in the off-gases as measured by GC was 3.3 percentby volume produced at a rate of 3.3 ml/min.

EXAMPLE 12

This example illustrates the use of elemental iodine as an oxidizingagent to make cuprous chloride/active.

A gaseous stream of 50 percent by volume butadiene in nitrogen was fedat a rate of 80 ml/min into the reactor of Example 1 containing 100 mlof an aqueous solution, having an initial pH of -0.2, which was 1 molarin cuprous chloride, 2 molar in sodium chloride, 0.3 molar in potassiumiodide and 0.5 molar in elemental iodine. During the reaction, thetemperature of the solution was maintained at 100° C. After one hour ofoperation, the gaseous product stream contained 45 percent by volumefuran as measured by GC, produced at a rate of 3.6 ml/min. Theconversion of butadiene was 10 percent and the yield of furan was 90percent.

EXAMPLE 13

This example demonstrates the effect of iodide on the rates of reaction.

A mixture of 80 percent butadiene in a mixture of butadiene and air wasfed at the rate of 100 ml/min to the reactor of Example 1 whichcontained a solution 2 molar in cupric chloride, 1 molar in cuprouschloride and 2 molar in calcium chloride. The pH of the reaction mixturewas 0.0 and the temperature was 95° C. After 45 minutes of operation,furan was produced at the rate of 0.8 ml/min. When 0.06 mole ofpotassium iodide was added to the reaction mixture, the rate of furanproduction increased until it reached 5.1 ml/min after another hour ofoperation. This is a 6.3-fold increase in the rate of furan production.

EXAMPLE 14

A stream of 36.5 ml/min of 17 percent 1,3-t-pentadiene in nitrogen wasobtained by sparging nitrogen at a rate of 30 ml/min through liquidtrans-1,3-pentadiene at 0° C. This gas stream was fed to the reactor ofExample 8 which contained 125 ml of a solution which was 2.4 molar incupric chloride, 1.4 molar in sodium chloride and 0.05 molar inpotassium iodide. The temperature in the reactor was 98° C. The gaseousproduct stream contained 0.5 percent of 2-methylfuran. The productstream was analyzed by gas chromatography and the identity of theproduct was established by comparison of the retention time with that ofan authentic sample.

EXAMPLE 15

A gaseous feed of 80 ml/min of 19.5 percentisoprene(2-methyl-1,3-butadiene) in 11 percent oxygen and 69 percentnitrogen was provided by vaporizing liquid isoprene at the rate of 0.075ml/min into a stream of 9.2 ml/min of oxygen and 54.0 ml/min ofnitrogen. The gaseous feed was introduced through a gas dispersion tubeto about 500 ml of an aqueous solution which was 1 molar in cupricchloride, 0.5 molar in cuprous chloride, 2 molar in calcium chloride,0.4 molar in hydrogen chloride and 0.025 molar in potassium iodide whichwas stirred in a 1-liter flask at 103° C. The chromatograph of theproduct stream showed that its components were 0.5 percent carbondioxide, 12.5 percent isoprene, 7.8 percent oxygen, 70.6 percentnitrogen and two new peaks. The reaction products were condensed in atrap cooled by dry ice-acetone. Analysis of the condensates by gaschromatography-mass spectroscopy identified one of the new peaks as 3-methylfuran.

EXAMPLE 16

A stream 100 ml/min of 50 percent butadiene in air was fed to a solutionof 300 g acetonitrile, 200 g water, 0.5 mole cupric chloride, 0.5 molecuprous chloride, 1.0 mole calcium chloride, 0.2 mole hydrogen chlorideand 0.0125 mole potassium iodide. The gaseous feed was introducedthrough a gas dispersion tube into the stirred solution in a 1-literflask which was heated at 80° C. After 2 hours of flow, analysis of theproduct stream by gas chromatography showed that it contained 0.32percent furan.

EXAMPLE 17

A feed of 0.07 ml/min of liquid crotyl chloride and a gaseous feed of150 ml/min of air and nitrogen were fed simultaneously to the reactor ofExample 3 which contained 400 ml of an aqueous solution 1 molar incupric chloride, 0.5 molar in cuprous chloride, 2 molar in calciumchloride, 0.06 molar in potassium iodide and 0.50 molar in hydrogenchloride. The reaction temperature was 95° C. The product stream fromthe reactor contained 3 percent furan and 9 percent butadiene.

EXAMPLE 18

1,4-Dichloro-2-butene (6.5 g, 0.05 mole) was introduced at the rate of0.05 ml/min to the reactor of Example 1 which contained 100 ml of anaqueous solution 1 molar in cupric chloride, 1 molar in cuprouschloride, 2 molar in calcium chloride and 0.13 molar in potassiumiodide. The pH of the mixture was 0.5 and the temperature was 95° C. Astream of 100 ml/min of nitrogen was passed through the reactor. Thegaseous product stream was passed through a trap cooled by a dryice-acetone bath. The liquid which was collected in the trap contained 7percent butadiene and 75 percent furan.

We claim:
 1. A reaction medium comprising(a) iodine from an alkali metaliodide, (b) copper having an average oxidation state between 1 and 2,(c) a solubilizing agent for cuprous ion which is soluble in water andforms a water-soluble complex with cuprous ion, and (d) at least 20moles per liter of water, the medium having a pH value of less thanabout
 2. 2. The medium of claim 1 wherein the solubilizing agent is analkali metal chloride, an alkaline earth metal chloride, or ammoniumchloride.
 3. The medium of claim 2 wherein the solubilizing agent issodium chloride, calcium chloride or ammonium chloride.
 4. The medium ofclaim 1 wherein the iodine is from sodium iodide or potassium iodide. 5.The medium of claim 1 wherein the copper is from cuprous chloride andcupric chloride.
 6. A reaction medium comprising(a) 1×10⁻¹² -0.5 grammole per liter of iodine from sodium iodide or potassium iodide, (b)0.1-10 gram moles per liter of copper from cuprous chloride and cupricchloride, the copper having an average oxidation state between 1 and 2,and (c) 0.01-5 gram moles per liter of sodium chloride, calcium chlorideor ammonium chloride,the medium having a pH value of less than about0.5.